1. Technical Field
The present invention relates to a bit change determination method or the like which determines the presence/absence of a change in a bit value of navigation message data.
2. Related Art
As a positioning system using a positioning signal, GPS (Global Positioning System) is widely known and used in a receiving device embedded in a mobile phone, a car navigation system, or the like. In the GPS, position calculation is performed to obtain the position coordinate and the timepiece error of the receiving device on the basis of information regarding the positions of a plurality of GPS satellites, the pseudo distance between each GPS satellite and the receiving device, or the like.
GPS satellite signals which are sent from the GPS satellites are modulated with spread codes, called CA (Coarse and Acquisition) codes, which differ between the GPS satellites. The CA codes are pseudo random noise codes having a repetition cycle of 1 ms with a code length 1023 chips as 1 PN frame. In order to acquire a GPS satellite signal from a feeble reception signal, the receiving device performs a correlation operation of the reception signal and a replica CA code which is a pseudo CA code generated inside the device, and acquires a GPS satellite signal on the basis of the obtained correlation value. In this case, in order to facilitate the detection of a peak correlation value, a method is used in which the correlation values are accumulated for a predetermined correlation accumulation time.
However, in the GPS satellite signals, the CA codes which are the spread codes are subjected to BPSK (Binary Phase Shift Keying) modulation in accordance with the bit values of navigation message data. For this reason, if the correlation operation of the reception signal when a GPS satellite signal is received and the replica CA code is performed, correlation values with exactly reversed signs are obtained before and after the bit values of navigation message data are changed (transited). Thus, when accumulating the correlation values over 20 milliseconds which are the data bit length of navigation message data, the correlation values having different signs may be accumulated.
In order to solve the above-described problem, for example, U.S. Pat. No. 5,768,319 describes a technique which calculates an inner product using a correlation value (hereinafter, referred to as “I-phase correlation value”) obtained by a correlation operation on an I-phase reception signal and a correlation value (hereinafter, referred to as “Q-phase correlation value”) obtained by a correlation operation on a Q-phase reception signal, and determines the presence/absence of changes in the bit values of navigation message data using the inner product value.
According to the technique of U.S. Pat. No. 5,768,319, the I-phase correlation values for 20 milliseconds are initially totalized to calculate an I-phase accumulated correlation value, and similarly, the Q-phase correlation values for 20 milliseconds are totalized to calculate a Q-phase accumulated correlation value. The inner product value is calculated using the I-phase accumulated correlation values and the Q-phase accumulated correlation values of adjacent 20-milliseconds periods. The inner product value calculated for every 20 milliseconds is accumulated for a predetermined time (for example, for 30 seconds), and the presence/absence of changes in the bit values of navigation message data is determined using the accumulation result of the inner product values.
According to the technique of U.S. Pat. No. 5,768,319, for bit change determination, it is necessary to accumulate and store data of the inner product values for a predetermined time in a memory. For example, if the inner product values for 30 seconds are used, since it is necessary to accumulate and store data of 1500 inner product values in the memory, the memory should be large in capacity. At the time of the initial reception of the GPS satellite signals, since bit change determination may not be made unless at least 30 seconds have elapsed, it takes a lot of time for the initial acquisition of the GPS satellite signals.